The collaboration of BioWare and Game Freak to create a Pokémon game with an amazing storyline. I'm thinking Mass Effect meets either one of HeartGold/SoulSilver or Black and White.
Things BioWare could add that Game Freak has yet to put in a Pokémon game:
- Voice-acting, dialogue wheels and a morality system. I would kill for characters to have voices in a Pokémon game. While morality is fairly obvious in the Mass Effect series (Paragon, Renegade, Boring But Reasonable) I like it and it works for me.
- An expanded story, maybe covering multiple regions of the Pokémon world. Naturally, there would have to be some degree of challenge to the other regions once you've beaten one of them.
- Closer camera shots, maybe similar to Mass Effect's third person and conversation views. Alternatively, first-person Pokémon would be awesome, switching to third person for dialogue.
- The ability to move in any direction, not just along the four compass points. That's something that Game Freak really needs to change in the next Pokémon games (because there will be more).
- Customisable characters. I would love to design my own Pokémon trainer, and possibly the models of the rival and/or companions (think Cheren and Bianca from B/W, only designed the way you want them). On a related note, allow characters to be aged anywhere between 16 and 21, player's choice.
- Romance subplots. I say subplots, plural, because where's the fun in just one?
Something else that would be suitably epic is a nice and strong replayability factor, taking into account character options such as morality and romances. Given that there are already 649 Pokémon (including legendaries and events) it shouldn't be so much of a buzzkill to allow people to obtain Pokémon from multiple generations. On that note, we should be able to pick a starter from any generation.